1. Field of the Invention
This invention relates to reinforced thermoplastic sheets and more specifically to warp-free multi-layer thermoplastic laminate sheets having both good surface and high strength qualities.
2. Discussion of the Prior Art
Various thermoplastic composite sheet materials are known in the art. For example, such thermoplastic polymers as polyolefins, polyamides, polyesters and the like can be reinforced with a myriad of reinforcing materials, examples of which include particulate fillers such as alumina, talc, clay, etc; flake-like fillers such as mica; and reinforcing fibrous material such as glass fibers, metal fibers, carbon fibers, etc. For example, U.S. Pat. No. 3,158,528 teaches thermoset and thermoplastic polymers reinforced with a combination of glass flakes, mica, and fibrous glass. Laminates prepared from such compositions are taught as being characterized by unusually low coefficients of linear thermal expansion. Also U.S. Pat. No. 3,684,645 teaches the reinforcement of a thermoplastic resinous sheet with relatively long glass fiber strands and relatively short glass fibers.
The presence of long glass fibers is particularly desirable to increase the toughness and impact resistance of the sheet. However, such long glass fibers lead to poor surface finish on the final product. For applications such as automotive exterior parts, appliance housings, furniture components, etc., a perfectly smooth, imperfection-free surface is mandatory. The measured depth of surface imperfections should be no more than 50-500 microinches (10.sup.-6 inch) when measured using a standard Bendix Corp. Micro-corder, profilometer or similar stylus-type profile indicator. A high concentration of long glass fibers near the surface of the finished product ordinarily leads to imperfections larger than this limiting value. Attempts have been made to solve this problem by locating the glass fibers adjacent to the non-decorative (unseen) surface of the sheet. However, this results in an uneven coefficient of thermal expansion of the sheet which is evidenced by warping of the sheet or products formed therefrom.
Other attempts have also been made to solve this surface finish problem. For example, U.S. Pat. No. 3,684,645 discloses a sheet comprised of a glass fiber strand mat confined with a thermoplastic polymer containing short glass fibers wherein the short glass fibers in the resin provide improved surface properties. Also, U.S. Pat. No. 4,044,188 discloses a smooth surfaced thermoplastic composite sheet containing at least one surface layer of thermoplastic polymer, particulate filler, and short glass reinforcing fibers and another layer containing thermoplastic polymer and relatively long or continuous glass fibers.
Although sheets and finished laminated products prepared according to these patents, as well as other prior art teachings are apparently smooth, they fail to pass the visual inspection test to which finished painted parts are normally subjected to in the automotive industry. One such test is to view the reflection of a fluorescent light on the painted part to determine if the surface has imperfections such as "orange peel" effects, roughness, waviness, ripples, etc. If the surface contains these imperfections, the part is rejected. Also, a problem has been found with "show-through" of the long glass fibers or mat which is again a visual test applied to the painted part.
Another problem associated with these prior art materials is warpage which is generally caused by grossly different average coefficients of linear thermal expansion (hereinafter referred to as CTE) for each layer over the operating temperature range of use. In order to overcome this warpage problem it has generally been the practice to prepare laminate sheets having an odd number of (at least three) layers so that the outer two layers could be used to balance the CTE on either side of a central core.
What is desired in the art however, is a two-layer laminated thermoplastic sheet rather than a three-layered sheet. Such a sheet would be more convenient, efficient and economical to manufacture. But warpage of such a sheet is an ever-present problem. This is due to the fact that a thermoplastic layer, such as nylon-6, containing various reinforcing aids, such as long fiberglass or mica exhibits one CTE below the glass transition temperature (T.sub.g) of the thermoplastic material, and another CTE above the glass transition temperature. Thus, the problem of matching the CTE's of two thermoplastic layers to form a warp-free laminated sheet is a more complex one than merely trying to match the CTE's of two different filled thermoplastic layers at a particular temperature.
A further complication was discovered by Leon Segal, published in Polymer Engineering and Science, April, 1979 Vo. 19, No. 5. Particulate filled nylon-6 compositions usually exhibit an .alpha..sub.1, the CTE below the T.sub.g, which is smaller in value than .alpha..sub.2, the CTE above the T.sub.g. For example, nylon-6, containing 20 percent by weight fine particle size kaolin, exhibits an .alpha..sub.1, of 5.93.times.10.sup.-5 .degree. C..sup.-1, from -30.degree. C. to T.sub.g (50-55.degree. C.), and an .alpha..sub.2 of 12.10.times.10.sup.-5 .degree. C..sup.-1, from T.sub.g to +170.degree. C. However, Segal discovered that weight loadings of long glass fiber above 10 weight percent in nylon-6, produced a lower .alpha..sub.2 than .alpha..sub.2, and further, the value of .alpha..sub.2 was very low. Since most materials exhibit a larger .alpha..sub.2 than .alpha..sub.1, matching a 10 percent by weight loading of long fiberglass in nylon-6, for example, with another layer of nylon-6, in which the CTE's were similar would be a difficult task.